The present invention relates to a microwave circuit and has particular, but not exclusive, application to a circuit fabricated on an RF printed circuit board (PCB).
Typically microwave circuits such as frequency converters for converting L-band to RF or RF to L-band include passive components such as filters. For convenience it is desirable if the filter could be defined as a pattern in the metal layer of the PCB. However, currently used commercial PCB patterning techniques are only able to define circuit tracks with 150 xcexcm gaps and track widths to a precision of xc2x120 xcexcm. This level of precision is too coarse to be able to define a filter having the performance required.
It is an object of the present invention to provide a microwave circuit having a desired performance on a PCB and in a cost effective manner.
According to one aspect of the present invention there is provided a microwave circuit comprising a printed circuit board consisting of a substrate having a metal backing on one surface of the substrate and at least one microstrip track on another surface thereof, and at least one sub-circuit mounted on the printed circuit board, the sub-circuit comprising a substrate having a higher dielectric constant than that of the printed circuit board substrate and having a conductive pattern thereon, the conductive pattern being coupled electrically to the at least one microstrip track.
According to another aspect of the present invention there is provided a microwave circuit comprising a printed circuit board on which is fabicated an RF stage having a frequency up-converter having an RF output coupled to a local oscillator reject filter and an image reject filter coupled to an RF input of a frequency down-converter, wherein the local oscillator reject filter and the image reject filter respectively comprise a sub-circuit comprising a substrate having a higher dielectric constant than that of the printed circuit board substrate and having a printed conductive pattern thereon, the conductive pattern being coupled electrically to the at least one microstrip track provided on the printed circuit board.
By virtue of the substrate having a higher dielectric constant than that of the printed circuit board, the sub-circuit can be physically smaller compared to a similar circuit made in PCB material. In an embodiment of the microwave circuit, the dielectric substrate comprises alumina which has a dielectric constant ∈r=9.9 compared to an ∈r=2.2 for the PCB material. The higher dielectric constant enables the sub-circuit on alumina to be half the size of that on the PCB material. The patterning technique can be done in a technology which is capable of achieving the required definition accuracy and which enables many hundreds of sub-circuits to be made simultaneously. Thus the method in accordance with the present invention enables two different technologies to be combined with a beneficial performance gain and cost effectiveness.